1. Field of the Invention
The present invention generally relates to a circuit for cancelling a waveform distortion of a video signal and, more particularly, is directed to a ghost cancelling receiver using a ghost cancel reference signal.
2. Description of the Prior Art
In the conventional television receiver, a ghost wave component is cancelled from the received video signal as follows.
That is, a ghost cancel reference signal (hereinafter simply referred to as a GCR signal) is added to the video signal in the transmitting side, while in the receiving side the GCR signal (involving the ghost wave component) of the received video signal and a GCR signal formed at the receiving side are compared with each other in waveform to thereby derive the ghost wave component and also a pass band characteristic of, for example, a transversal filter is controlled so as to cancel the thus produced ghost wave component.
The GCR signal at that time may be a signal SGCR shown in FIGS. 1A through 1C.
Throughout FIGS. 1A to 1C, reference symbol HD represents a horizontal synchronizing (sync.) pulse and BRST a burst signal. As shown in FIG. 1A, a first GCR signal WRB has a bar-shaped waveform located at the rear side of the horizontal period and a width of 44.7 microseconds. The level of the first GCR signal WRB is 70 IRE and the rising characteristic thereof is a ringing characteristic of sin X/X.
Further, a second GCR signal PDS has a pedestal waveform (0 level) as shown in FIG. 1B.
As shown in FIG. 2A, 8 field periods of the video signal are used as the repetitive cycle and the signal WRB is inserted into 18th line or 281th line of the first, third, sixth and eighth field periods. Also, the signal PDS is inserted into 18th line or 281th line of the remaining second, fourth, fifth and seventh field periods and the video signal having the GCR signal inserted thereinto is transmitted.
If the calculation shown in FIG. 2B is carried out in the receiving side under the condition such that the first to eighth GCR signals SGCR are taken as S.sub.1 to S.sub.8, then the calculated result provides a signal GCR shown in FIG. 1C. Further, if the video signal has a ghost, then this signal GCR contains a ghost wave component Sg.
Therefore, the ghost can be cancelled from the signal GCR (and Sg) of the calculated result.
In that case, the burst signal BRST, the color signal and the horizontal synchronizing pulse HD, which are distant by 4 field periods, are the same in phase so that, when the signals S.sub.1 to S.sub.8 are calculated, the burst signal BRST, the color signal and the horizontal synchronizing pulse HD.
Accordingly, since the signal GCR (and the ghost wave component Sg) of the calculated result does not contain the burst signal BRST, the color signal and the horizontal synchronizing pulse HD, so-called pre-ghost and delay-ghost can be eliminated and the waveform can be equalized in a range of 45 microseconds at maximum. Further, a long ghost of about 80 microseconds can be detected without error.
Furthermore, the GCR signals sequentially transmitted during 8 field periods are employed as one set and one ghost wave component Sg is obtained from the above one set by the calculation shown in FIG. 2B so that, even when the received GCR signal SGCR contains a noise component, such noise component can be suppressed and the S/N ratio can be improved by 3 dB.
FIG. 3 shows an example of a conventional ghost cancelling circuit which utilizes the GCR signal.
As shown in FIG. 3, there is provided a video detector circuit 1 for a television receiver. The video detector circuit 1 derives a color composite video signal SY added with the above-mentioned GCR signal SGCR. The color composite video signal SY is supplied to an analog-to-digital (A/D) converter 2, in which it is converted into a digital video signal SY whose one sample is formed of, for example, 8 bits. The digital video signal SY is supplied through a transversal filter 3 having, for example, 640 stages (640 taps) to a digital-to-analog (D/A) converter 4, in which it is converted into the original analog video signal SY. The analog video signal SY is developed at a terminal 5.
In that case, a detection circuit 10 detects a ghost wave component from the GCR signal SGCR and the pass band characteristic of the transversal filter 3 is successively corrected by a detected output of the detection circuit 10, thereby the ghost wave component being eliminated as described above.
That is, the calculation shown in FIG. 2B can be rewritten as the calculation shown in FIG. 2C, which means that the GCR signals SGCR of respective field periods are sequentially added.
Accordingly, the digitized video signal SY from the transversal filter 3 is supplied to a gate circuit 11 from which there is derived a GCR signal SGCR (including preceding and succeeding detection periods). The thus produced GCR signal SGCR is supplied to a buffer memory 12 which then stores the GCR signal SGCR of the field period at every field period.
The GCR signal SGCR of the buffer memory 12 is supplied to a calculating circuit 21. Although the calculating circuit 21 and the succeeding circuits 22 to 25 are formed of a microcomputer 20 and a software in actual practice, they are represented by the hardware in FIG. 3.
In the calculating circuit 21, the GCR signals SGCR held in the buffer memory 12 are sequentially added or subtracted in accordance with the equation shown in FIG. 2C to thereby produce the signal GCR and the ghost wave component Sg which are the calculated results of the 8 field periods. The signal GCR and the ghost wave component Sg are both supplied to a subtracting circuit 22 and a signal GCR of reference waveform (see FIG. 1C) is generated from a reference GCR signal generator circuit 23. This GCR signal is supplied to the subtracting circuit 22.
Accordingly, the subtracting circuit 22 derives the ghost wave component Sg of the received signal GCR and this ghost wave component Sg is also the error component from which the ghost cannot be eliminated.
Since the signal GCR has the bar-shaped waveform, the signal GCR must be arranged to have a pulse waveform. For this reason, the ghost wave component Sg is supplied to a differentiating circuit 24, in which it is differentiated to provide a differentiated pulse Pg. The differentiated pulse Pg is supplied to a converting circuit 25.
In the converting circuit 25, the differentiated pulse Pg is converted into a signal ST indicative of a correcting amount of a tap coefficient (tap gain) of the transversal filter 3. The signal ST is supplied to the transversal filter 3 to control the pass band characteristic of the transversal filter 3 so that the PG,6 ghost wave component Sg involved in the GCR signal SGCR is eliminated.
Thereafter, the above-mentioned processing is repeatedly performed so that the pass band characteristic of the transversal filter 3 is adjusted gradually, that is, the pass band characteristic of the transversal filter 3 is successively corrected. Thus, the pass band characteristic of the transversal filter 3 is converged gradually to the characteristic which eliminates the ghost wave component Sg of the GCR signal SGCR.
When the pass band characteristic of the transversal filter 3 is converged sufficiently, then the level of the ghost wave component Sg of the GCR signal SGCR is decreased to a negligible level. At that time, the level of the ghost wave component of the original video signal SY is also decreased to a negligible level. Therefore, the video signal SY from which the ghost wave component is eliminated is developed at the terminal 5 (see "GHOST CANCEL REFERENCE SIGNAL SYSTEM" on Journal of 1989 Nationwide Conference held by The Institute of Television Engineers of Japan).
If the GCR signals SGCR of 8 field periods are added to produce the ghost wave component Sg, then the noise component can be suppressed and the S/N ratio can be improved by 3 dB.
However, it is frequently observed that the noise component cannot be suppressed sufficiently by the above-mentioned processing in the area of weak electric field. If the tap coefficient of the transversal filter 3 is determined under the condition such that the noise component is not suppressed sufficiently, then the noise component and the ghost wave component Sg cannot be discriminated from each other, which causes an error to occur in the detection of the ghost wave component Sg, resulting in the ghost wave component being added to the signal.
In order to solve the above problem, the following proposal is made. That is, the S/N ratio of the received GCR signal SGCR is detected and when the S/N ratio is not satisfactory, the ghost wave component of one field period is calculated not from one set (8 field periods) of the GCR signal SGCR but from a plurality of sets of continuous GCR signals SGCR. According to this proposal, the duration of period in which the GCR signals SGCR are averaged is extended so that the noise component can be eliminated more reliably, thus making it possible to eliminate the ghost wave more reliably.
In accordance with the above proposed method, however, it takes a long period of time to suppress the noise component sufficiently so that the transversal filter needs a lot of time to converge its pass band characteristic, thus decreasing the speed at which the ghost is cancelled.